1. Field of the Invention
The present invention relates to thermal mass flow sensors.
2. Description of the Related Art
Circuits for mass flow sensors have long used a bridge configuration in measuring the mass flow rate of a fluid. The common bridge configurations included a servo constant temperature configuration, such as Hubbard, U.S. Pat. No. 4,843,881, and a constant current configuration. The bridge configurations were included as a part of a feedback network or servo loop.
The servo loop of the constant temperature configuration has required numerous current paths to be driven by a differential amplifier. That differential amplifier was used to drive the current path including an inductive heating and sensing coil for heating the fluid and sensing the mass flow rate of the fluid. Servo loop control of the sensing coil was undesirably influenced by the power requirements for the numerous current paths that did not drive the sensing coil. The power requirements of some of these current paths were at current levels on the order of 50 to 80 mA. Other of these current paths were typically on the order of 0.1 to 0.5 mA. As can thus be appreciated, these current paths typically carried high levels or amounts of current.
The constant current configuration has required even higher levels of current typically on the order of 160 mA. In the constant current bridge, the current paths were more interdependent than the current paths present in the servo loop of a constant temperature configuration. The high levels of current present in both configurations were unsuited to sensing coils used for fast temperature sensing responses.
Further, sensors using bridge configurations have required sensing coils in arms of the bridges to have the same coil parameters in order to achieve optimum sensing performance. Typical coil parameters included the number of coil turns, the electrical resistance of the coils, and the temperature coefficients of change of the coils. Lack of uniformity of these coil parameters has been a significant limitation on the performance of mass flow sensors using bridge configurations. Coils with even slightly differing parameters have significantly reduced the performance of such mass flow sensors.